1. The Field of the Invention
The present invention relates to a neural network system, and more particularly to a switched-current type of a Hamming neural network system for pattern recognition with an improved performance and increased reliability for the Hamming neural network system.
2. Description of the Related Art
Currently, an artificial neural network has become a fashionable and popular research topic. The neural network theory is thought to be the way with the highest potential for solving many artificial intelligence problems. However, because a software neural network can not satisfy many applications requiring a real-time processing, a design of a very large scale integrated circuit (VLSI) neural network has become an important research approach.
Due to a high connection density among neurons in many neural networks and a requirement of connection priority values to be flexibly controlled, it is difficult to construct a network including a larger number of neurons by using a VLSI technique. The construction of a Hamming neural network is simple and the value implying the connection priority can be set in advance and then applied to the VLSI circuit in the network in many applications. Therefore, the Hamming neural network has become an essential construction in the integrated circuit neural networks.
In pattern recognition applications, a typical Hamming neural network is of a two-layer construction. A circuit for computing a matching rate between a to-be-identified pattern and a standard pattern is formed on the first layer and a circuit for finding a maximum network, that is, a Winner-Take-All (WTA) network which is used to find a corresponding standard pattern having a maximum matching rate and outputting it as a result is formed on the second layer. Although this typical Hamming neural network is easily constructed, it can only find out the standard pattern which is the one that is most matchable with the to-be-identified pattern. However, with the raising in the complexity of the system, the increasing in the number of the standard patterns, and especially the development of a stage-connected system with multiple stages, this is far from satisfying the performance requirement of the system. To raise the performance of the system, it is very necessary to find two or more standard patterns which are most closely matchable with the to-be-identified pattern based on the matching rates between the standard patterns and the to-be-identified pattern. Therefore, a new developed switched-current Hamming neural network is set forth hereinafter, in which the matching rates can be outputted according to their ranks from high to low and the corresponding standard patterns. Thus, m standard patterns which are the most and more closely matchable with the to-be-identified pattern can be found sequentially where 1.ltoreq.m.ltoreq.N. This will greatly improve the system performances such as the increase of the identification rate, the data reprocessing and reuse of the multi-stage stage-connected system.
Generally, a threshold value is taken for increasing the reliability of the system. This threshold value is a threshold value set in a decision-made phase of the system and is divided into an absolute threshold value and a relative threshold value. Two different threshold values correspond to two identification-rejection (IR) methods which are an absolute IR method and a relative IR method. The absolute IR method indicates that the system produces an IR signal if the maximum matching rate is less than the absolute threshold value, and the relative IR method specifies that the system generates an IR signal when a difference value between the maximum matching rate and a second maximum matching rate is less than the relative threshold value. Since the typical Hamming neural network can only perform a maximum-finding operation, it can merely carry out an absolute IR judgment but can not make a relative IR judgment. However, performing the relative IR judgment in many practical applications will more greatly increase the reliability of the system than that of the absolute IR judgment. Thus, the present invention discloses an IR circuit which has a simple construction and a good performance and makes both the absolute IR and the relative IR judgments.